Known copying materials utilizing photosensitive diazo compounds are divided into three types. The first one is known as a wet development type, which comprises a support having provided thereon a photosensitive layer comprising a diazo compound and a coupling component. This material in intimate contact with an original, is exposed to light and developed with an alkaline solution. The second one is known as a dry development type, which is different from the wet development type in that ammonia gas is used in place of the alkaline solution. The third one is known as a heat development type and includes a type of material in which a photosensitive layer contains an ammonia gas-generating agent capable of generating ammonia gas upon heating, such as urea; a type in which a photosensitive layer contains an alkali salt of an acid which loses acidic properties upon heating, such as trichloroacetic acid; and a type in which a photosensitive layer contains a higher molecular weight fatty acid amide as a color forming assistant which activates a color forming reaction between a diazo compound and a coupling component upon heat-melting.
Disadvantages associated with the use of a wet development type copying material include the necessity of replenishment or disposal of the developing solution, maintenance of a large-sized apparatus and inability to write on the copies immediately after removal from equipment because of wetness. In addition, the reproduced image is not stable, i.e., does not withstand long-term storage.
The dry development type copying material also involves the use of developing solution and thus, the disadvantages include those associated with the use of a wet development type. In addition, other disadvantages include the requirement of large-sized gas absorption equipment for preventing leaks of ammonia gas, and the copies immediately after removal from the equipment smell of ammonia.
On the other hand, the heat development type copying material is advantageous in that equipment maintenance is minimum because no developing solution is used. Nevertheless, any of the state-of-the-art heat development type copying materials require high temperatures ranging from 150.degree. to 200.degree. C. for development. Moreover, the developing temperature must be controlled within 10.degree. C. of the prescribed temperature. Otherwise, the development becomes insufficient or the tone of the reproduced image is changed. Therefore, the apparatus cost is high. In addition, the diazo compound must be highly heat resistant for use in high temperature development, and such heat-resistant diazo compounds are disadvantageous in that high density images are not obtained. Attempts to develop such materials at low temperatures (90.degree. to 130.degree. C.) have resulted in copying materials having a reduced shelf life.
Thus, in spite of the fact that heat development type copying materials are advantageous as to equipment maintenance over the wet or dry development types, they still have significant disadvantages and thus are not routinely used.
In the heat development type copying materials comprising a support having provided thereon a photosensitive layer containing a diazo compound, a coupling component, and a color forming assistant, each of the components must be melted, diffused and reacted with heat to form a dye before a desired color density can be obtained. Assuming that a copying material which would undergo a color formation reaction at a low heating temperature to obtain a high density image could be successfully designed the problem of the color formation reaction taking place while the material before copying is held at room temperature may still be encountered. Should the reaction occur, the background of the copying material, which should be white, becomes colored.
The inventors conducted extensive studies in order to solve the above-described conflicting problems. As a result, they found that encapsulization of at least one, of the diazo compound and the coupling component, is a basic solution.
The developed color density of heat development type copying materials is determined by the quantity of heat energy given and the amounts of color-forming components. High efficiency of heat conduction from a heating means to the copying material and uniformity of dye formation through the heat conduction are essential factors for obtaining a high quality copied image particularly as the developing temperature is lowered. In other words, if heat conduction is non-uniform due to the unevenness of a copying material, the dye formation is uneven, and unevenness of the image density may result. Further, if the color-forming components are localized due to the unevenness of a support, the dye formation similarly becomes uneven, resulting in unevenness of image density. It is therefore highly desirable to avoid such image density unevenness.
The microcapsules previously proposed for the purpose of satisfying both shelf life and heat sensitivity of heat development type copying materials are susceptible to the unevenness of the support surface because they exist as fine particles in a film. When using paper as a support, the microcapsules may penetrate into the support to cause non-uniform dye formation. Therefore, problems including uneven density of the copied image remain unsolved even with the copying materials using the microcapsules.